Headlight covers - as in modern-car plastic headlamps? If the fog is on the smooth portion (the outside) you can try a product called "MicroMesh" (it's available in a small kit). MM is used for restoring plastic aircraft windows, and is essentially just a kit full of small pieces of abrasives and polishes. It will restore an olmost opaque airplane window to like-new condition, I would think it would do the same on car plastics.